import boto
import boto.ec2
import time
regions = boto.ec2.regions()
eu = regions[0] #eu-west-1
conn_eu = eu.connect()
rs = conn_eu.get_all_instances(instance_ids='<your instance id here>')
reservation = rs[0]
instance = reservation.instances[0]
#start an existing instance
instance.start()
print instance.state
while (instance.state != 'running'):
    time.sleep(5)
    instance.update()
    print 'pending ...'
print instance.ip_address